The present invention relates to methods of utilizing solar ponds for controlling the temperature of solutions. The invention is particularly applicable to the use of solar ponds in various industrial processes for promoting the dissolution of salts in a solution and/or the controlled precipitation of salts from a solution. When used for precipitating salts, the invention may also be applied for controlling the nature or form of the salts precipitated, for example, to obtain dehydrated salts or salts of predetermined crystal sizes, through the control of the solution temperature and the rate of temperature change.
The control of temperature, and the rate of its rise or fall in large bodies of salt solutions, is of great importance in some industrial processes, but in many cases it is not economically attainable. It is particularly important in processes involving dissolution or precipitation of salts whose solubility varies considerably with temperature. In most cases, the solubility of salts in water increases with temperature. However, in some cases, the solubility decreases with temperature, and in still other cases, it increases with temperature in one range and decreases in another. Points of discontinuity in the relation between solubility and temperature usually coincide with a change in the hydration of the salts precipitating out of the solution.
In the precipitation of salts whose solubility varies rapidly with change in temperature, the supersaturation of the solution required to cause precipitation is often obtained not by evaporation of the solvent, but rather by a change in its temperature, or by both. Also, the dissolution rate of such salts can be increased considerably by a suitable change in the solvent temperature.
In some industrial processes not only the temperature is important, but also the rate of its variation. This rate is particularly important inthe precipitation of solutes when the product is required in the form of large compact crystals with a minimum of fine powder or skeleton crystals which are easily crushed into powder. Economically important examples of such products are potash and other solid fertilizers which are increasingly required today for reasons connected with modern application methods in the form of 2 to 4 mm. compact crystals, the market price for which is considerably higher than the price for their powder form.
It is well known that one condition for the formation of large compact crystals by precipitation from a supersaturated aqueous solution of hetropolar salts is that the rate of crystal growth must be slow. The rate of crystal growth is dependent on temperature and the composition of salts in the solution. That is, it is proportional to the degree of supersaturation of the solution around it which, in turn, depends on the rate of change of temperature, and/or the rate of solvent evaporation provided, of course, that the contact between the supersaturated solution thus obtained and the growing crystals is continuously maintained.
Because of the slow rate of crystal growth required for large-size crystals, very large volumes of solution must usually be processed for normal production capacities, and therefore the use of industrial crystalizers often becomes uneconomical. The only known method in which a very slow rate of crystal growth would not be economically prohibitive, is by the use of EP's, (evaporation ponds). However, insofar as we are aware, no method has yet been devised, applicable to the natural conditions, for adequate control of crystal size so that some salts (e.g. sodium chloride) are often obtained in the form of large crystals which are later ground while other salts (e.g., potasium chloride) required in the form of large crystals are precipitated in EP's as skeletons and small crystals which, in many cases, have to be later pelletized in expensive pelletizing plants to meet market demand for larger particles.
According to the present invention, solar ponds are utilized for controlling the temperature of solutions in order to accomplish many industrial processes including those mentioned above.
The term "solar pond" is commonly used to describe two types of ponds which are heated by solar radiations. The first type is known as an evaporation pond (or EP), in which various salt solutions are concentrated and/or salts are precipitated by the use of solar radiations for evaporating solvent from the pond. The second type is commonly called a non-convective solar pond (or NCSP) in which various salt solutions are heated to temperatures up to 100.degree. C. and over, by establishing a stable concentration or density gradient which produces stable temperature gradient increasing from the top of the pond to its bottom. The bottom layer in the pond, which is usually darkened and of the highest salt concentration, absorbs the solar radiations reaching it while the loss of heat upwards is greatly reduced (in the absence of convection currents) because of the existence of the density gradient. NCSP's have been used mainly as large solar energy collectors wherein the heated bottom layer is circulated and utilized outside of the pond.
NCSP's have also been suggested for the dehydration of sodium sulphate and other salts in the pond. In this case, the suggestion is to add feed solution to the top of the pond, the bottom of the pond being heated by solar radiations to a temperature higher than the dehydration temperature while solvent is evaporated from the top, resulting in the precipitation of salt at the bottom of the pond where it is dehydrated. But, in this technique the whole pond becomes saturated. Such a pond can be maintained as an NCSP only when the density of the saturated solution continues to increase with temperature also at temperatures higher than the dehydration temperature, which is not the case with sodium sulphate (See FIG. 7). Furthermore, since the rate of evaporation of water from salt solution ponds decreases with the increase in concentration, the efficiency of utilizing solar energy for the evaporation of water from completely saturated ponds is usually low.
However, the NCSP can be used for trapping solar energy in the bottom layer and at the same time it can utilize additional solar energy as an evaporation pond in which the top layer of more dilute solution is concentrated, as long as the required density gradient in the non-convective layer of the pond is maintained.
The present invention uses NCSP's both as dissolution ponds to increase the salt concentration of a solution, and as precipitation ponds to precipitate salt from a solution. In both cases, the pond may be used for either raising or lowering the temperature of the solution for increasing or for decreasing the solubility of the solute, in accordance with the characteristics of the particular solution involved as discussed above. In addition, when the pond is used for precipitation, the temperature and the rate of its change may be controlled to determine the nature (e.g., dehydrated) and particle size of the precipitate. In both cases, the NCSP can also be used at the same time as an evaporation pond.
A third type of pond, which may be used in some applications of the present invention is the "controlled temperature evaporation pond" (or, CTEP). In the commonly used EP absorbing heat from solar radiations only, the temperature of a solution circulated through the pond is determined by the climatic conditions (i.e., ambient temperature), and is nearly independent of the pond size. However, when energy is added to the pond by introducing the solution at a high temperature and withdrawing it at a lower temperature, then the temperature of the pond can vary anywhere between the temperature of the introduced hot solution for a very small pond, to the ambient temperature for a very large pond. The surface area of the pond required to maintain a predetermined temperature of a given solution between the said limits, for a given circulation rate and climatic conditions, can be approximately calculated. In practice, the climatic conditions are not constant. The temperature of the pond will also fluctuate with the diurnal climatic cycle and other variations in the climatic conditions. However, if these variations during the same season are not extreme, and the pond is deep enough (a few meters), the solution temperature fluctuations remain within a few degrees Centigrate of the required temperature. Such a pond can be used, therefore, as an evaporation pond in which evaporation is carried out at an approximately predetermined temperature which is higher than the ambient temperature.
A more accurate control of temperature, and adjustment of the pond surface area to seasonal variations in climatic conditions, can be obtained by the use of a floating cover having a surface area predetermined according to the required temperature.